The present invention is directed to high intensity discharge (HID) lamps, and more particularly to compact HID lamps with short arc lengths that are mated with a reflector.
Compact HID lamps are used for a variety of purposes, including projecting video and data and injecting light into an optical light guide. Various types of compact HID lamps are known, for example see U.S. Pat. Nos. 5,691,601 and 4,686,419, which are incorporated by reference. All of these lamps have two electrodes whose separation defines the arc gap in the lamp. Compact HID lamps have an arc gap on the order of one to several millimeters.
Compact HID lamps are not without problems. The amount of light is less than expected. While there are hot spots immediately in front of the electrodes where high luminance regions exist, much light comes from the less intense, remainder of the arc gap. As arc gap length is reduced, light from the latter source is reduced. Further, as the size of the arc gap is reduced, lamp efficiency deteriorates because of heat build-up. In addition, the electrodes cause shadows and interfere with optical coupling. In an effort to reduce shadowing, the electrodes are often tapered. This can cause very high electrode tip temperatures and melting.
An object of the present invention is to provide a novel HID lamp that avoids one or more of these problems by using only one electrode sealed in the lamp""s light transmissive envelope.
Another object of the present invention is to provide a novel HID lamp that includes a sealed, light transmissive envelope, a powered, high intensity discharge-producing electrode sealed within the envelope, and a ground electrode outside the envelope.
A further object of the present invention is to provide a novel HID lamp that includes a starting gas, a vaporizable fill, and only one electrode sealed within a light transmissive envelope, where the lamp is mated with a reflector that has an electrically conductive surface that is a ground for electric field lines emanating from the powered electrode during operation of the lamp.
A yet further object of the present invention is to provide a novel method of operating a HID lamp by applying high frequency power to a powered electrode that is the only electrode sealed in the lamp""s light transmissive envelope to cause an arc discharge in the envelope, and grounding electric field lines from the electrode to a conductive surface that is adjacent to the envelope.